Salt stress, caused by high levels of fertilizer salts in the growing medium, can greatly inhibit the growth of bedding plants. Salt accumulation in the root zone can affect plants because of osmotic stress, ionic imbalances, or specific nutrient toxicities (Dubey, 1996). Researchers previously have reported that higher than recommended leachate electrical conductivity (EC) can reduce plant growth (Gislerød and Mortensen, 1990; James and van Iersel, 2001; Kang and van Iersel, 2001; Nemali and van Iersel, 2004; van Iersel, 1999; van Iersel and Kang, 2002). In general, the leachate EC increases with increasing fertilizer concentrations and these effects get more pronounced over time as fertilizer salts accumulate in the growing medium (van Iersel, 1999). Subirrigation may result in more problems with the accumulation of soluble salts in the growing medium than overhead watering, because excess salts are not leached from the growing medium (Kent and Reed, 1996).
Even when constant fertilizer concentrations are used, the growing medium EC will generally change over time because of such factors as changes in the frequency of fertigation and the balance between water and nutrient uptake by the plants. Environmental conditions that increase evapotranspiration such as low relative humidity or high temperatures but have less effect on plant nutrient uptake will increase salt accumulation in the growing medium unless fertigation practices are adjusted. For example, Kang and van Iersel (2001) found that the optimal fertilizer concentration for petunia growth decreased with increasing growing temperature. At high relative humidity, dry weight of begonia (Begonia ×hiemalis Fotsch) was found to be higher at higher fertilizer concentrations (Gislerød and Mortensen, 1990). High relative humidity reduces the air-to-leaf vapor pressure deficit and thus the transpiration rate, increasing water use efficiency (Dewar, 1997). In general, environmental conditions that affect the water use efficiency of plants may also affect the optimal fertilizer concentration (Bugbee, 1995; Nemali and van Iersel, 2004). Therefore, fertilizer recommendations for bedding plants based on constant fertilizer concentrations may not be optimal; nutrient availability to the plants depends more on the amount of nutrients in the growing medium than on the amount of nutrients in the fertilizer solution.
Traditionally, recommendations for fertilization of bedding plants have focused on which concentration of fertilizer to apply. Recently, the focus of fertilization guidelines has shifted from recommending optimal fertilizer concentrations to maintaining the EC of the growing medium or the leachate within an optimal range (e.g., Cavins et al., 2000; Kang and van Iersel, 2002). Warncke and Krauskopf (1983) reported that a growing medium EC [as determined by the saturated medium extract method (SME)] of 0.75 to 2 dS·m−1 is acceptable, whereas 2 to 3.5 dS·m−1 is optimal for most greenhouse crops. Cavins et al. (2000) reported that a leachate EC of 1.0 to 2.6 dS·m−1 (as determined by the pourthrough method) is optimal for annuals with low nutrient requirements, whereas 2.0 to 3.5 dS·m−1 is optimal for greenhouse crops with medium nutrient requirements.
Growing medium-based guidelines have the advantage that they are less dependent on environmental conditions than optimal fertilizer concentrations. The optimal EC of the growing medium or leachate is likely to be much less sensitive to environmental conditions, including temperature, photosynthetic photon flux, and humidity, than the optimal fertilizer concentration (Gislerød and Mortensen, 1990; Kang and van Iersel, 2001; Nemali and van Iersel, 2004). Fertilizer recommendations based on the leachate EC give a better indication of the nutrient availability to the plants as well as the potential for salt stress than recommendations based on fertilizer concentrations. However, the disadvantages of trying to maintain a constant leachate EC include the requirement for regular and time-consuming measurements of leachate EC and the need to periodically adjust the fertilizer concentration to maintain the leachate EC within the required range.
Surprisingly, there has been no research comparing the effects of constant fertilizer concentration and constant leachate EC on the growth of floriculture crops. Therefore, the objective of this experiment was to determine whether maintaining a constant leachate EC results in better growth of petunia and wax begonia than constant fertilizer concentrations.
Argo, W.R. & Biernbaum, J.A. 1996 Availability and persistence of macronutrients from lime and preplant nutrient charge fertilizers in peat-based root media J. Amer. Soc. Hort. Sci. 121 453 460
Bailey, D.A. & Bilderback, T. 1997 Alkalinity control for irrigation water used in nurseries and greenhouses Hort. Info. Lflt. 558. North Carolina Coop. Ext. Serv
Bugbee, B. 1995 Nutrient management in recirculating hydroponic culture 15 30 Proc. 16th Annu. Conf. on Hydroponics Hydroponic Soc. Amer San Ramon, CA
Cavins, T.J., Whipker, B.E. & Fonteno, W.C. 2005 Timing of PourThru affects pH, electrical conductivity, and leachate volume Commun. Soil Sci. Plant Anal. 36 1573 1581
Cavins, T.J., Whipker, B.E., Fonteno, W.C., Harden, B., McCall, I. & Gibson, J.L. 2000 Monitoring and managing pH and EC using the pourthru extraction method Hort. Info. Lflt. 590. North Carolina Coop. Ext. Serv Raleigh, NC
Dubey, R.S. 1996 Photosynthesis in plants under stressful conditions 859 875 Pessarakli M. Handbook of photosynthesis Marcel Dekker, Inc New York, NY
Gislerød, H.R. & Mortensen, L.M. 1990 Relative humidity and nutrient concentration affect nutrient uptake and growth of Begonia ×hiemalis HortScience 25 524 526
Havlin, J.L., Beaton, J.D., Tisdale, S.L. & Nelson, W.L. 2004 Soil fertility and fertilizers. An introduction to nutrient management Prentice-Hall Upper Saddle River, NJ
James, E.C. & van Iersel, M.W. 2001 Fertilizer concentration affects growth and flowering of subirrigated petunias and begonias HortScience 36 40 44
Jones, J.B. & Case, V.W. 1990 Sampling, handling, and analyzing plant tissue samples 389 427 Westerman R.L. Soil testing and plant analysis Soil Sci. Soc. Amer Madison, WI
Kang, J.G. & van Iersel, M.W. 2001 Interactions between temperature and fertilizer concentration affect growth of subirrigated petunias J. Plant Nutr. 24 753 765
Kang, J.G. & van Iersel, M.W. 2002 Nutrient solution concentration affects growth of subirrigated bedding plants J. Plant Nutr. 25 387 403
Kent, M.W. & Reed, D.W. 1996 Nitrogen nutrition of New Guinea impatiens ‘Barbados’ and Spathiphyllum ‘Petite’ in a subirrigation system J. Amer. Soc. Hort. Sci. 121 816 819
Lang, H.J. 1996 Growing media testing and interpretation 123 139 Reed D.W. Water, media, and nutrition for greenhouse crops Ball Publishing Batavia, IL
Mills, H.A. & Jones, J.B. 1996 Plant analysis handbook. II. A practical sampling, preparation, analysis, and interpretation guide MicroMacro Publishing Athens, GA
Nemali, K.S. & van Iersel, M.W. 2004 Light intensity and fertilizer concentration: I. Estimating optimal fertilizer concentrations from water-use efficiency of wax begonia HortScience 39 1287 1292
van Iersel, M.W. 1999 Fertilizer concentration affects growth and nutrient composition of subirrigated pansies HortScience 34 660 663
van Iersel, M.W. & Kang, J.G. 2002 Nutrient solution concentration affects whole-plant CO2 exchange and growth of subirrigated pansy J. Amer. Soc. Hort. Sci. 127 423 429
Warncke, D.D. & Krauskopf, D.M. 1983 Greenhouse growth media: Testing and nutrition guidelines Michigan State Univ. Coop. Ext. Serv. Bul. E
Yeager, T.H., Wright, R.D. & Donahue, S.S. 1983 Comparison of pour-through and saturated pine bark extract, N, P, K, and pH levels J. Amer. Soc. Hort. Sci. 108 112 114